Thin film fabrication techniques are becoming essential aspects of many advanced technology manufacturing operations. In particular, thin film semiconductor fabrication techniques have been used for decades. Newer techniques employing gallium arsenide ("GaAs") and other semiconductor materials are being developed to exploit the high speed and other superior performance properties of such materials. Recent advances in superconducting devices made from high critical temperature ("T.sub.c ") ceramic compounds have necessitated development of new processes for fabricating, annealing and testing exotic materials. Such processes include pulsed laser ablation, single target sputtering, multi-element co-sputtering, metallo-organic chemical vapour deposition ("MOCVD"), molecular beam epitaxy ("MBE"), electron beam co-evaporation, reactive sputtering and etching, and various other processes.
Many of these processes involve chemical reactions, partial pressures, etc. which are highly temperature dependant. Accordingly, the temperature of the thin film fabrication process must be carefully controlled. For example, thin films are conventionally fabricated at operating temperatures ranging from about 600.degree. C. to about 900.degree. C., with the operating temperature being ideally controlled within a tolerance range of about .+-.1.degree. C. to .+-.5.degree. C. In practice however, the absolute operating temperature cannot economically be measured with the desired precision, so a relative temperature is used instead. Thus, the temperature is often controlled by reference to a "tracking" variable such as a thermocouple readout, with the optimal process temperature being determined empirically by evaluating the quality of successive batches of finished thin films and iteratively adjusting the associated value of the tracking variable until acceptable quantities of thin films of acceptable quality are produced.
A further problem is that it is difficult to reliably obtain a good thermal connection between the thermocouple and the substrate on which the thin film is fabricated. Consequently, the absolute value of the temperature of the substrate may be known only to within about .+-.10.degree. C. Thus, even if it is possible to control the process temperature to within .+-.1.degree. C., such accurate temperature control resolution can not be applied effectively. More importantly however, the fabrication of uniform quality thin films over the entire substrate requires that the temperature of the substrate be uniformly controlled over the entire substrate. The larger the substrate, the more stringent this control must become and the more difficult it is to attain.
In most epitaxial deposition processes currently in use, the substrate on which thin film structures are to be fabricated is mounted in a support holder which is attached to a thermally massive heater block. A good thermal connection to the block is achieved by using a heat conducting medium such as indium solder or silver paste to bond the substrate to the block. This causes two major problems. First, the bond is often non-uniform across the substrate. Second, after fabrication of the thin film structures on the substrate, the bond must be broken to free the substrate from the heater block (i.e. by physically prying the substrate away from the block).
These problems impose production limitations which inhibit widespread use of thin film fabrication techniques in comparison to other techniques such as those used to fabricate silicon devices. For example, a non-uniform bond between a 50 mm diameter substrate and the heater block may cause temperature variations of several degrees across the substrate, making it impossible to fabricate thin film structures of uniformly high quality over the entire substrate. The fragility of the substrate material results in significant breakage loss when the bond is broken to remove the substrate from the block. Breakage loss escalates rapidly as the diameter of the substrate increases above about 50 mm. Large diameter substrates are preferably utilized in order to maximize yield and minimize cost, but the breakage factor limits the size of substrates which can practically be accommodated in many fabrication processes.
A further problem is that the mechanical bonding of one side of the substrate to the heater block renders that side of the substrate unsuitable for thin film fabrication. This precludes double-sided thin film fabrication of the sort commonly encountered in microwave device fabrication, which greatly constrains the design of the circuits capable of fabrication by thin film techniques.
The foregoing problems have restricted the use of various thin film fabrication processes, limited the yields attainable by those processes, and added greatly to their complexity and expense. However, if the foregoing problems can be overcome, many of these processes offer significant potential benefits in the fabrication of thin film microcircuits. The present invention is accordingly directed to overcoming such problems.